It is possible to analyze video sequences of a living subject and detect small changes in the images which are the result of physiological processes of that subject. Amongst these physiological process are such things as blood flow, breathing and sweating.
Certain physiological processes can be observed via skin reflectance variations. The human skin can be modelled as an object with at least two layers, one of those being the epidermis (a thin surface layer) and the other the dermis (a thicker layer underneath the epidermis). A certain percentage 5% of an incoming ray of light is reflected at the skin surface. The remaining light is scattered and absorbed within the two skin layers in a phenomenon known as body reflectance (described in the Dichromatic Reflection Model). The melanin, typically present at the boundary of epidermis and dermis, behaves like an optical filter, mainly absorbing light. In the dermis, light is both scattered and absorbed. The absorption is dependent on the blood composition, so that the absorption is sensitive to blood flow variations. The dermis contains a dense network of blood vessels, about 10% of an adult's total vessel network. These vessels contract and expand according of the blood flow in the body. They consequently change the structures of the dermis, which influences the reflectance of the skin layers.
Other physiological processes such as breathing cause movement in the surface of patient.
Other physiological processes such as variations in blood oxygenation level can manifest themselves as small colour changes.
It is possible to detect and extract signals which have some periodic content in these changes and from that obtain a result such as a frequency in the case of periodic processes. For example, a subject may be illuminated with ambient light and filmed using a video camera. By analyzing changes in the values of corresponding pixels between frames of the sequence of images, a time-variant signal can be extracted. This signal may be transformed into frequency-like domain using something like a Fast Fourier Transform and from the frequency-domain spectra, a value for the subject's heart-rate may be arrived at as a physiological measurement. These physiological measurements are often called vital signs.
The changes in the pixel values are often small and often more pronounced in 1 colour channel than the others. Thus the signal that is being looked for is correspondingly small.
There may be other changes in the pixel values such as those due to changes in the general image and these can be comparatively large. There are also sources of random change in the pixel values such as noise in the image sensor and variations in the illumination (such as flicker). All of these are, to all intents and purposes, uncorrelated with the signal being sought. Therefore the signal to noise ratio is small and the physiological measurement may be sometimes of questionable reliability.